Mass spectrometry



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inventors Graham 6. Wonless Purenf Arrorney United States Patent r 3,234,379 MASS SPECTRGMETRY Graham G. Wanless, Westfield, and George A. Glock, Jr., South Plainfield, NJ., assignors to Esso Research and Engineering Company, a corporation of Delaware Filed May 31, 1963, Ser. No. 284,541 2 Claims. (Cl. 250-413) This invention relates to mass spectrometry and particularly relates to means and methods for increasing the resolving power of a mass spectrometer. More parparticularly, it relates to an improvement in singlefocusing mass spectrometers which use electron bombardment for providing an ion source. Modern versions of this ion source are frequently referred to as Nier-type sources.

Mass spectrometry is one method of determining the chemical or elemental composition of a material. In the conventional Nier-type mass spectrometer, the material to be examined is introduced into an ionization chamber and subjected to electron bombardment. The ions generated therein are accelerated into a mass tube by an electrostatic field and are resolved into successive groups of individual mass by the dispersing power of a magnetic field. The relative abundance of each mass group is then measured and recorded in the shape of peaks on an oscillographic chart. Every ionized mass group has a distinct mass-to-charge ratio and therefore should appear as a separate peak on the oscillographic record. In the case of two ions which difier by as much as one unit of mass, the spectrometer does produce separate and distinct peaks. However, if the masses of the two ions approach the same figure, the peaks reproduced on the oscillographic record also approach each other. When the difference in mass becomes so small that the mass spectrometer is unable to resolve the ions into separate recordedpeaks, the potential mass doublet is said to be unresolved.

The resolving power of a mass spectrometer may be described as the degree of separation which can be obtained between two peaks in a mass spectrum. Consider, for example, the parent peaks of N-nonane and 2-octanone. Nominally, both molecules weigh 128 atomic mass units (a.m.u.). However, their exact masses diifer by a fraction (AM), which is equal to 0.0364 a.m.u. Complete resolution of these two peaks is said to occur if a resolving power equal to M/AM is achieved. In the example, the requirement for complete resolution is M/AM=128/0.0364==3516. This means that, if a mass spectrometer has a resolving power of 3516, a 0% valley will exist between the two recorded peaks of N-nonane and Z-OCtHnOll. A lesser value will produce overlapping of the recorded peaks and result in only partial resolution.

Partial resolution can be gauged by the depth of the valley between the peaks when the peak shapes are symmetrical and triangular. This is possible because the depth of the valley between two overlapping but identical isosceles triangles is proportional to the degree of overlapping.

Many methods for improving the resolving power of mass spectrometers have been proposed and tried. It is known that increased resolution can be achieved by the reduction of chromatic aberration within the instrument. However, the sources of chromatic aberration are many and varied. It has now been found that chromatic aberration within a single-focusing mass spectrometer can be minimized by operating the ion source circuit with the quietest available direct current, i.e., battery power. It has been further found that the insertion of a capacitor at a very particular location in the high voltage power supply will further substantially increase the resolving power beyond that achieved by the use of the direct current modification mentioned above.

It is therefore an object of this invention to minimize chromatic aberration within a single-focusing mass spectrometer in order to obtain increased resolving power. It is also an object of this invention to provide the art of mass spectrometry with an improved and relatively simple ion source circuitry, whereby the increased resolving power may be obtained. In this manner, a singlefocusing mass spectrometer can be utilized for high resolution studies as well as for conventional work.

The invention and its objects may be more fully understood from the following specification when it is read in conjunction with the accompanying drawings in which:

FIGURE 1 is a schematic illustration of a simplified conventional single-focusing mass spectrometer;

FIGURE 2 is a schematic illustration of a modified form of a portion of the apparatus such as shown in FIGURE 1, wherein a voltage divider has been added and the A.C.-filament control circuit is replaced by a completely D.C.-powered filament control circuit; and

FIGURE 3 is a schematic illustration of a section of the high voltage power supply showing the positioning of an additional capacitor.

Referring now in greater detail to FIGURE 1, a schematic diagram of a simplified conventional mass spectrometer is shown. The type illustrated employs the conventional A.C.-regulated filament control system 10. This equipment sustains the emission of electrons at a near constant rate by causing the filament 14 to burn more or less brightly. The amount of current transmitted to the filament through lines 8 and 9 is a function of the amount of electric current received at the trap 15 The trap is incorporated into the control circuit by a suitable lead 16.

The material to be analyzed is admitted into the i0n ization chamber 20 by means of suitable inlet 18. For convenience, the ionization chamber 20 may alsobe referred to as the shield. There, the sample is subjected to the electron beam 40 emitted from the filament 14. The ions thereby produced are expelled from the ioniza tion chamber by means of stimuli provided by a repeller plat-e 21 and an ion gun assembly 22. The repeller plate is maintained at a slightly higher positive potential than the ionization chamber in which it is situated. This slight difference in potential is sufiicient to impel the ions toward the ion gun slit 24. Within the .ion gun assembly 22 are two collimatiiig slits 25 and 26 and a series of accelerating electrodes 28, 29 and 30, which are held at successively decreasing voltage levels. As a result of the large voltage drop across the accelerating electrodes, ions entering the ion gun assembly are propelled into the mass tube 31 at high speeds. 7

As the accelerated ionized particles travel through the mass tube, they are resolved into separate homogeneous beams such as 42, 43 and 44 by means of a magnetic field produced between the poles of an electromagnet 32. The strength of the magnetic field is a function of the current flowing through the winding 33. At any given strength, only that homogeneous ion beam, e.g., beam 43, whose mass-to-charge ratio corresponds to the strength of the magnetic field can negotiate the curved section of the mass tube and pass through the collector slit 27. The ions passing through the collector 3 slit impinge on the collecting electrode 35 and produce a small current. This current is amplified and recorded by means of suitable apparatus 11. The oscillographic chart produced by the recording apparatus 11 indicates the extent of resolution.

As previously mentioned, chromatic aberrations within the mass spectrometer have a pronounced effect on the resolving power of the instrument. An important source of chromatic aberration is the A.C. ripple imposed on the electron beam by the A.C.-regulated filament control circuit. This ripple is in addition to and is superimposed on the finite'energy distribution which exists in the electron beam itself. The sum of these two effects causes the positive ions which are formed by electron bombardment to acquire a finite energy distribution about the mean. As a result the emerging ions have a finite velocity distribution, whereas ideally they should have no such distribution. This aberration results in a loss of resolution to the instrument.

The A.C. ripple described above is not to be confused with the well-known A.C. wavelet which is imposed on the ion beam by cyclic oscillation in the stabilized high voltage power supplywhich creates the eletcrostatic accelerating voltages. 7

It has now been discovered that'the forementioned A.C. ripple can be reduced by replacing the A.C.-regulated filament control circuit with a simple arrangement of A and B batteries. By quieting the operation of the ion source, ions having narrower distributions of energy are obtained and, as a consequence, the resolving power of the mass spectrometer is increased about 51%. The embodiment of this first improvement is represented by FIGURE 2.

In the system illustrated by FIGURE 2, those parts and elements which correspond to parts previously illustrated by and described with reference to FIGURE 1 are designated-by the same numerals. Thus much of the detailed description of the purposes and functions of those parts and elements need not be repeated. The differences may be discovered in the ion source circuitry and in the addition of the voltage divider which was not shown in FIGURE 1. As shown in the drawing, the filament 14 is now powered by-a set of 6-volt A batteries 50 arranged in parallel. This arrangement is used to provide sufiicient capacity so that no essential change in current output occurs during an 8-hour shift. Current to the filament is regulated by a hand-operated control 13 associated with a rheostat 12. The trap 15 is incorporated into the circuit by line 76 which is composed of leads 76a, 76b and 76c. A B" battery 51, interposed in line 76, is connected to the shield by line 66 and functions to establish the filament-to-shield potential. Lead 66a connects the positive side of B battery 51 to potentiometer 48. Potentiometer' 48 is connected to the shield by lead 66b and regulates the filament-to-shield potential. In general, high resolution is favored by a relatively low filament-to-shield potential. However, the filament-to-shield potential is only one among many spectrometric conditions which aifect resolution. The optimum value is dependent upon the instrument and the other spectrometric conditions. In general, it will vary from about 15 volts to about 55 volts. A second set of B batteries 52 is interposed in line 76 in series with the first B battery 51 so as to establish a trap-to-shield potential of between 45 to 270 volts and thus the repeller plate current.

The voltage divider 55 is suitably connected to a source of high voltage, not shown, and to ground 85. For purposes of illustration, a total voltage of about 5500 volts is used to indicate the relative voltage drops across the ion gun assembly 22. As shown, the repeller plate 21 is connected by lead 58 to the voltage divider at point 57. Similarly, the shield is connected by a suitable lead 60 at point 61. Since the repeller plate is held at a slightly higher positive potential than the shield,

a voltage differential of 3 volts is shown. Accelerating electrode 28 which operates as a draw-out plate is con nected by lead 64 to the voltage divider at point 63. The voltage to ground at this point is about 5070 volts. Both sides of electrode 28 are maintained at the same voltage by interconnecting lead 65. One half of accelerating electrode 29 is connected by lead 68 to the voltage divider at point 67. The voltage at this point is about 610 volts. The other half of accelerating electrode 29 is linked by lead 70 at point 71. The voltage to ground at this point is about 570 volts. One half of the last accelerating electrode 30 in the ion gun is connected by lead 74 to the voltage divider at point 73. The voltage at this point is about 90 volts. The other half of accelerating electrode 30 is connected by lead 75 to electrical ground 85. Collimating slits 25 and 26 are attached to ground by respective leads 77 and 78.

Despite the improvement illustrated in FIGURE 2, wherein the conventional A.C.-regulated filament control circuit was replaced with sources of direct current free of A.C. ripple, it was found that there was still an oscillation associated with the ion source. The prior art suggests that this oscillation is a result of an A.C. wavelet superimposed by the stabilized high voltage power supply. However, it has been discovered by extensive experimental circuit modifications that this particular oscillation is not a superimposed A.C. wavelet, but is a phenomenon associated with the operation of the ion gun itself. It has further been discovered that this oscillation can be removed or greatly reduced by installing an additional capacitor in the high voltage line between the shield and electrical ground. Experimentation has shown that only a capacitor inserted at this particular juncture improves the operation of the ion gun. Capacitors ranging in size from 0.1 to 1.0 mfd. (13,000 v.), preferably 0.2 'mfd., may be employed to filter out the aforementioned oscillation. When this further modification is incorporated into a circuit such as illustrated by FIGURE 2, the resolving power of the mass spectrometer is increased approximately an additional 52%. In accordance with the present invention, the embodiment of this second improvement is schematically represented by FIGURE 3.

In order to simplify FIGURE 3, only a portion of the voltage divider is shown. Those parts and elements which correspond to parts previously illustrated and described are designated by the same numerals. As shown in the drawing, the additional capacitor 62 is connected tothe voltage divider at point 60 by lead 79, and to electrical ground 86 by lead 80.

The exact nature and advantages to be derived from the present invention may be more fully understood by referring to the following examples. However, it should be understood that the examples are not to be construed as limiting the invention in any way.

EXAMPLE 1 Collimating slits, in. 0.002 Collector slit, in. 0.0005 Accelerating voltage, v. 5500 Filament to shield potential, v. 45

These and all other minor parameters are the same in both runs. The results of an analysis of the mass spec trum generated are tabulated in Table I. They show that the circuit modifications of this invention effect a marked increase in resolving \power.

5. Table I Percent Valley Between Peaks of Mass 58 Douhlet Estimated Resolving Power Critical Factor In I Percent Instrument Design Increase All regulated filament control system D.C. ion source circuitry defined by Figure 2 The depth of the valley between the peaks of the mass 58 doublet are used to measure the degree of resolution.

This is possible because of the excellent triangular peak shapes which are achieved. If two peaks, which differ only by a fractional mass, are of the same height, they will have the same width at the base and will approximate two identical isosceles triangles. The depth of the valley between two such identical triangles is directly proportional to their degree of overlapping.

For example, if two identical isosceles triangles are arranged side by side so that the base of one occupies one-half the base of the other, there is a 50% overlapping. In that situation, the adjoining sides of the triangles intersect at their midpoints and form a valley. The depth of that valley is equal to one-half the height of the isosceles triangles and signifies a 50% valley or 50% resolution.

6 The above results establish that the combination of the circuit modifications defined by FIGURES 2 and 3 elfectively eliminate part of the chromatic aberration due to superimposed AC. ripple and to oscillation attributed to the ion gun. As a consequence, resolving power is markedly improved.

EXAMPLE 3 An equal volume blend of N-nonane and 2-octanone is placed in a conventional 12" radius, -60" single-focusing mass'spectrorneter. After an initial run, the spectrometer is adapted to conform with the circuit modification defined in FIGURE 3 and another-run made. Upon completion of the second run, the spectrometer is .further adapted to conform with the circuit modifications "defined in FIGURE 2 and a third run made. The most important constant mass spectrometric conditions for all runs are:

All other minor parameters are the same as in Examples 1 and 2. The results of an analysis of the mass 43 doublet generated are tabulated in Table III below.

Table III IMPROVED MASS SPECTROMETER REsorLvsIlhgG POWER DUE TO CHANGES IN CIRCUIT Valley Percent 4 Left Hand Right Hand Between Valley Estimated Critical Factor In Instrument Peak Value Peak Value Peaks of Between Resolving Percent Design (Units) (Units) Mass 43 Peaks of Power Increase Doublet Mass 43 (Units) Doublet A.C.-regulated filament control system 700 322 319 99 ll. 5 A.C.-regulated filament control system plus additional capacitor as defined by Figure 3 c 740 375 123 32. 8 775 6, 640 D.C. ion source circuitry defined by Figure 2 plus additional capacitor as defined by Figure 3 3,070 1, 794 225 12. 5 1, 008 30 IMPROVED MS. RESOLVING POWER DUE TO IMPROVEMENT IN ION SOURCE DESIGN Percent Valley Estimated Percent Critical Factor in Between Resolving Improve- Iustrumeut Design Peaks of Power ment In Mass 71 Res. Power Doublet D.C. ion source circuitry defined by Figure 2 32 1, 250 Above, plus circuit modification of Figure 3 2 1, 900 52 The left hand peak of the mass 43 doublet is a measurement of the lighter mass group of the doublet and the right hand peak corresponds to the heavier mass group. Determination of the valley between the left and right hand peaks is made by measuring the distance from the bottom of the valley so formed down to the base of the oscillographic images. This figure is compared to the shorter peak in order to arrive at the percent valley. The figures in the above table are relative values since no attempt Was made to achieve complete resolution or equal oscillographic peak heights. The above results further illustrate that increased resolving power is achieved by the circuit design changes of the present invention.

EXAMPLE 4 Equal volumetric amounts of N-nonane and 2-octanone are mixed and samples from the mixture are charged to a single-focusing mass spectrometer equipped with the improvements defined in FIGURES 2 and 3 and to a CBC-2 l-1l0 double-focusing mass spectograph. Ten runs were made on each machine and the average values for mass 43, 71 and 128 calculated. The results are compared With the absolute mass values in Table IV.

Table I V.

ABSOLUTE MASS :M'EASUREMENTS Correct 3 IF. Mass S.F. Mass 95% Number Spectro- Spectrom- Confidence graph eter Limit 2 -3222 was l. 072 1. -----@-a-- use; lit-2232 12 D 15 u -0. 0011 -a 0012 From Table IV it is seen that the improved single-focusing mass spectrometer of the present invention gives correct results Well Within the 95% confidence limit and is as accurate as the more eflicient and advanced doublefocusing mass spectrograph.

I What is claimed is:

1. In a mass spectrometer ion source employing a stabilized high voltage power supply having an alternating component for creating electrostatic accelerating voltages across an ion gun assembly and a shield, the improvement which comprises a capacitor, said capacitor being electrically connected and being essentially the only electrical element between the shield and electrical ground and being of sufficient size to suppress an inherent ripple due to the alternating component within the ion gun assembly.

2. An apparatus according to cl im 1 in which the capacitor is rated at 0.2 mfd.

References Cited by the Examiner UNITED STATES PATENTS RALPH G. NILSON, Primary Examiner. 

1. IN A MASS SPECTROMETER ION SOURCE EMPLOYING A STABILIZED HIGH VOLTAGE POWER SUPPLY HAVING AN ALTERNATING COMPONENT FOR CREATING ELECTROSTATIC ACCELERATING VOLTAGES ACROSS AN ION GUN ASSEMBLY AND A SHEILD, THE IMPROVEMENT WITH COMPRISES A CAPACITOR, SAID CAPACITOR BEING ELECTRICALLY CONNECTED AND BEING ESSENTIALLY THE ONLY ELECTRICAL ELEMENT BETWEEN THE SHIELD AND ELECTRICAL GROUND AND BEING OF SUFFICIENT SIZE TO SUPRESS AN INHERENT RIPPLE DUE TO THE ALTERNATING COMPONENT WITHIN THE ION GUN ASSEMBLY. 